The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The known method for rendering harmless fibrous building materials, like asbestos, which comprises heating it till it reaches the temperature of about 1400° C., close to its melting temperature, by contacting it with fumes of a traditional furnace. As a result, asbestos fiber meltdown and fading of its crystal structure takes place and asbestos is transformed into a safe non-fibrous structure.
An example of the traditional method can be found in the international patent application published as WO 91/00123. It discloses a process for the mineralogical conversion of waste asbestos to a non-asbestos product. The waste asbestos is treated with a mineralizing agent and then heated for a period of time sufficient to convert the asbestos to the non-asbestos product. The mineralizing agent is preferably a water-soluble alkali metal borate or alkali metal silicate.
The process of heating is carried out in a conventional oven until the material achieves a temperature above 2000° F. Due to isolation properties of some materials containing asbestos, there is needed a long time of heating such materials. For this reason, energy outlay to effectively utilize (making harmless) asbestos is very large. Introduction of additional substances in WO 91/00123 has as its aim to boost the asbestos fibres destruction as the result of thermochemical processes. The publication presents only laboratory examples which illustrates the difficulties to adapt the disclosed process to commercial applications in the industrial scale.
A German patent specification DE 19526495 A discloses another method and a device designed for thermal modification of building materials, mainly asbestos, binded during creation of hydrates. This method comprises treating fibrous materials with microwave radiation inside a processing chamber, with wave length ranging from 10-1 m to 10-6 m, whereas at least a part of the radiation fits into a range of maximum absorption, as a result the materials are heated so much that their fibrous structure disintegrates.
The processed material is introduced inside the chamber in portions and heated up to the desired temperature in a certain time dependant on radiation power. A device for realizing such a method has a processing chamber to which a microwave generator is attached having at least one radiator emitting microwave radiation. The processing chamber has a loading inlet and an unloading outlet which are close by sealing gates, and the whole device has a special shield preventing the microwave energy spreading. Placing radiators opposite to each other, as well as rotating the processed material, is very advantageous. At least one radiation beam penetrates into the processing chamber of the device, the said beam emitted by parabolic antennas and directed crosswise to the chamber axis. Another embodiment of such a device comprises a container with a processing zone, across which the processed material is moved. A microwave generator with a radiator is connected to the container, said radiator creating microwave radiation in the processing area. Processing chamber has an inlet and an outlet through which processing material is being inserted and taken out. Both inlet and outlet are closed by air locks and the whole device has a protective shield.
Because materials containing asbestos poorly absorb microwave radiation, maximum absorption during their processing can be reached only within a narrow range of microwave length and frequency. Effective heating of materials, merged as a result of the creation of hydrates, is achieved by the use of radiation in the band of so called polar molecule chemically bounded resonant absorption. It corresponds to a frequency of over 4 GHz, and infrared radiation. In industrial practice it is permitted to use microwave generators with frequencies 2.45 GHz and 915 MHz, which correspond to wave length 12, 24 centimeters and 32.77 centimeters accordingly. Of course, in case of need one can use generators with other, optimally selected, frequencies and respectively bigger power; however they are very expensive and almost inaccessible. Moreover, the temperature of about 1400° C., which is necessary to disintegrate the fibrous structure of asbestos, with known methods of its thermal processing, requires huge amounts of energy. Additionally, radiators in the form of parabolic antennas, used with generators of the known device, can be used only for emitting the non-polarized microwave field. Placing radiators opposite to each other brings only partial success, because the maximum field intensity has a local range. Due to this fact we can obtain an effective heating only in a small area of the converted material. A significant disadvantage of non-polarized radiation is a risk of harmful coupling between generators having radiators situated opposite to each other. This effect of generator coupling can become even more serious when the emitted electromagnetic waves reach a frequency at which there is weak field absorption by the processed material.
In practice, the method acc. to DE 19526495 A cannot be used, as one cannot use the microwave generators for frequencies different than approved for industries, i.e. 2450 MHz and 915 MHz, among others. Further, high power generators for frequencies above 3 GHz are difficult to obtain and very expensive—they are used only in scientific research.
The European Patent Application EP 1 588 783 teaches a process for inactivating industrial waste including crystalline or semi-crystalline material containing H2O or crystal water, which consists in subjecting the industrial wastes to high power microwaves to explosive like convert the H2O or crystal water into the vapor phase and to destroy its solid state structure.
It teaches also an apparatus having generally a chamber with input, output and conveying means, and a microwave source. It does not disclose any specific method as such nor a definite construction of an apparatus, so it is not suitable for the direct industrial application. It is taught there that the material should not be crumbled and its subject matter consists on strong local overheating (“hot spot”). Strong local overheatings cause uneven heating of the material. As the result, some part are having lower temperature and they do not react sufficiently (some areas with fibrous asbestos structure may remain).
It is equally important, that by the methods acc. to DE 19526495 A and EP 1 588 783 it is not possible to make harmless the asbestos contained in asbestos cloth, asbestos ropes or different asbestos boards, as such waste do not contain water or watery silicon compounds.